1. Field of Invention
The present invention relates to a power-switching device. More particularly, the present invention relates to a power-switching device capable of directing power from either a battery or an external power source to an all-time operational system such that the working life of the battery can be extended.
2. Description of Related Art
In modern life, we are surrounded by various kinds of electronic products. However, not every one of these electronic products is in use twenty-four hours a day. Nevertheless, for some electronic equipment, part of its internal circuitry may need to be functional even when it is not switched on. For example, the clock circuit within a person computer has to be maintained even when power supply to the entire computer is switched off.
Therefore, the next time the computer is powered up, the clock circuit can still provide accurate time to the computer. In addition, almost all personal computers have memory circuits for storing some important setup parameters. The memory circuit keeps important parameters for operating peripheral devices such as hard disks or floppy disks. At present, the memory circuits and the clock circuit are integrated together on a single chip, known often as the real-time clock (RTC) integrated circuit (IC). In some systems, the RTC IC is even integrated within a silicon chip module for reducing device counts. The aforementioned clock circuit and memory circuit, which must be operational even when the main power supply is cut off, is called an all-time circuit. In other words, an all-time circuit is operational whether surrounding circuits are operational or not.
Since all-time circuits need to remain operational after the main power supply is switched off, two sources of power including an external power source and a battery are normally simultaneously provided. When the external power source is tapped, working power of the all-time circuits is provided by the external power source alone. Consequently, current flow from the battery is reduced to a minimum so that battery power is retained. On the other hand, when main power to the all-time circuit is cut off, the battery takes over and provides all the necessary power for operating the all-time circuit. For example, when a personal computer is switched off, power for operating the RTC IC is provided by an on-board battery.
FIG. 1 is a block diagram showing a conventional circuit connection between an external power source, a battery and an all-time circuit. In FIG. 1, input line VCC is connected to an external power source, and line VBAT is connected to a battery 130. When line VCC is not connected to an external power source, power for operating the all-time circuit 110 is provided by the battery 130. Since the diode 120 is forward biased when an external power supply is absent, the battery 130 is able to supply the necessary power via the line VBAT.
However, when the external power supply is connected to the line VCC, power to the all-time circuit 110 is provided by the external power source. The external power source is able to take over because an external power source generally has a higher voltage than the battery 130. Thus, the diode 120 becomes reverse bias and hence is able to stop any flow of current from the battery 130. Because the recharging of a rechargeable battery may lead to environmental problems, non-rechargeable batteries such as lithium batteries are often used as the background power source. In general, for non-rechargeable batteries, forced charging not only may damage the battery itself, but may also lead to a shortening of its working life. With the diode 120 becoming reverse bias when the external power source is connected, the flow of current from the power line VCC into the battery 130 is prevented.
Although the circuit connection from the external power source and battery as shown in FIG. 1 is able to provide a constant supply of working power for the all-time circuit, potential drop still exists between the terminals of a diode due to a forward bias. Consequently, power stored inside the battery cannot be fully utilized. FIG. 2 is a graph with a curve showing the variation of battery voltage with discharging time. Assuming a brand new battery can provide a maximum voltage of V0, battery voltage gradually drops with time due to discharging.
For example, at time t1, battery voltage has dropped to about V1. There is a certain voltage range for the battery within which the all-time circuit can operate when the external power supply is cut off. However, when the battery voltage has dropped below a limiting value, the all-time circuit does not function. For example, the all-time circuit RTC IC in a personal computer can normally work when a powering voltage higher than 1.8V is provided, but the RTC IC ceases to function below 1.8V. In general, the voltage of a new battery for supplying RTC IC is about 3.0V, that is, V0=3.0V. Supposing a diode has a forward bias drop of about 0.7V when conductive, the RTC IC stops functioning when battery voltage has dropped to about 1.8V+0.7V=2.5V. On the other hand, if the RTC IC is directly connected to the battery instead of indirectly through a diode, the RTC IC is still operational as long as the battery voltage is still above 1.8V.
In FIG. 2, if the battery is connected via a diode and the value of V1, V2 are 2.5V and 1.8V respectively, the RTC IC stops working after t1 because the battery voltage has dropped below 2.5V. However, if the battery is directly connected to the RTC IC instead of via a diode, the RTC IC can still function normally before time t2 at about 1.8V. In other words, without a diode between the battery and the RTC IC, the battery can work an extra period from t1 to t2. If the battery within a personal computer for working the RTC IC has a life of more than two years, the working life of a battery can be extended for a few more months.
In summary, the conventional method of using a diode to provide battery power to an all-time circuit has the disadvantage of producing a voltage drop at the terminals of a diode when the diode is in forward bias. Therefore, the all-time circuit stops operating even when the battery voltage is still above the minimum operating voltage of the all-time circuit. Hence, the battery is not fully utilized.
In light of the foregoing, there is a need to provide a method of extending the working life of a battery.